In the past, as semiconductor devices were scaled down in size, thinner and thinner oxides were required for proper operation of the smaller and smaller transistors. The reductions in oxide thickness were achieved by reduction in oxidation time, decreases in oxidation temperature, and decreases in oxygen flow or concentration during processing of the semiconductor wafer.
This approach has been successful as the semiconductor devices were scaled down from micron to submicron levels. As the semiconductor devices are scaled down to the sub-100 nanometer range, even thinner oxides are required and it is apparent that there are limits to how much the oxidation time can be reduced, the oxidation temperature decreased, and the oxygen flow decreased.
At one minute of oxidation time, a maximum oxidation temperature of 800.degree. C., and low oxygen flow, which are the practical limits of the various parameters, the theoretical limit of the thinnest oxide is still greater than 10 angstrom. Further reductions in time, increases in temperature, or reductions in oxygen flow eliminate manufacturing reproducibility and make the oxide thickness undependable.
Thus, there appear to be practical limits on the ability to scale down semiconductor devices using the conventional approach.
For sub-100 nanometer semiconductor devices, it is absolutely necessary that the transistor gate oxide thickness be below 10 angstroms in thickness. And there is apparently no way of reaching this thickness.